The present invention relates to a method for the preparation of a composition which contains potassium salts of complex fluorides of aluminum and which is suitable as a flux for soldering lightweight metallic materials such as aluminum. A further aspect of the invention relates to a reaction mixture which can be used to produce such a flux.
In hard soldering or furnace soldering, metallic materials, particularly aluminum, are joined at a temperature above 500.degree. C. with the aid of a molten auxiliary metal (solder). The melting temperature of the solder is lower than the melting temperature of the materials, so they can often be separated without destroying them by re-melting the solder.
During soldering, oxides and other interfering cover layers on the metal surface are a problem: the surface must be metallically pure, so that a satisfactory soldered joint is produced. For this purpose, fluxes are used, which are usually brushed or sprayed on or are applied to the material as a coating.
Fluxes composed of potassium salts of complex aluminum fluorides are particularly suitable as fluxes.
U.S. Pat. No. 4,428,920 (=EP 063,750) describes a method for preparing a flux of this type. According to that method, a potassium compound, preferably potassium hydroxide in the form of a potassium lye solution, is added to fluoroaluminic acid in a precipitation stage whereupon potassium salts of complex fluorides of aluminum are precipitated. Then the water content of the suspension of potassium salts of complex fluorides of aluminum produced in this precipitation stage is separated, and the resulting solids are dried at 120.degree. C. In this method, a flux is obtained which melts without residue below 575.degree. C.
In that method, advantageously freshly prepared fluoroaluminic acid (obtainable from aluminum hydroxide and hydrofluoric acid) is used. The potassium compound is used in a stoichiometric shortage; this means that fluoroaluminic acid is still present in the aqueous supernatant of the resulting complex aluminum fluoride.